Your tuck is the most important thing when it comes to aerodynamics and keeping speed. I've worked on mine and have the progression in the photos. Here's some guidelines;
Put your knee as low as you want, if it makes keeping your chest down comfy, do it.
Keep your chest to your knee.
Keep your butt down.
Keep your arms rounded.
Be comfortable and try to keep your tuck aero at the same time
REMEMBER: Everyone has a different body shape, don't copy my tuck because it might not be right for you. Play with it and find te right position for you.
Feel free to message me with questions!
Stay steezy,
-Patrick